Pressure-sensitive adhesives (PSAs) for application to the skin or other body surface are well known and have been used for many years in a variety of consumer and medical applications. Pressure-sensitive adhesives are characterized as being normally tacky and exhibiting instant tack when applied to a substrate. Many polymers have been used to manufacture pressure-sensitive adhesives as, for example, acrylic and methacrylic ester homo- or copolymers, butyl rubber-based systems, silicones, urethanes, vinyl esters and amides, olefin copolymer materials, natural or synthetic rubbers, and the like. All the PSAs are elastomers, i.e. they exhibit viscoelastic properties typical of rubbers.
Existing examples of pressure-sensitive adhesives used for affixing a drug delivery system, cushion, or the like to the skin include polysiloxanes (e.g., polydimethyl siloxanes, polydiphenyl siloxanes, and siloxane blends), polyisobutylenes, polyacrylates, acrylic acid-acrylate copolymers (e.g., copolymers of acrylic acid copolymers with 2-ethylhexyl acrylate or isooctyl acrylate), and tacky rubbers such as polyisobutene, polybutadiene, polystyrene-isoprene copolymers, polystyrene-butadiene copolymers, and neoprene (polychloroprene). All of these PSAs are hydrophobic polymers and their common disadvantage is a loss in adhesion toward hydrated substrates.
“Bioadhesion” is defined as a pressure-sensitive adhesion with respect to highly hydrated biological tissues such as mucosal tissue. In contrast to conventional pressure sensitive adhesives such as rubber, polysiloxanes, and acrylates that adhere mainly to dry substrates, bioadhesives (BAs) exhibit good tack when adhered to hydrated biological substrates. To be bioadhesive, water should provide a plasticizing effect on a polymer, i.e., the polymer should be hydrophilic. For example, the range of typical BAs includes slightly cross-linked polyacrylic and polymethacrylic acids (EP 0 371 421) as well as blends of hydrophilic cellulose derivatives (40–95%) with polyethylene glycol (PEG) (U.S. Pat. No. 4,713,243).
Bioadhesives become tacky as the crosslinked polymer swells in significant quantities of water. The cohesive strength of highly swollen hydrophilic polymers is generally low and the BAs thus differ from the PSAs in this regard.
Attempts to combine the properties of PSAs and BAs have been described by Biegajski et al. in U.S. Pat. No. 5,700,478, where a water-soluble pressure-sensitive mucoadhesive was obtained by blending 95–40% polyvinylpyrrolidone (PVP) with 0–50% hydroxypropyl cellulose (HPC) and 11–60% glycerol. Other examples of hydrophilic polymer blends coupling the properties of PSAs and BAs involve polyacrylic acid-polyvinyl alcohol (PAA-PVA) interpolymeric complexes formed by hydrogen bonding between the monomer units of the complementary polymer chains and plasticized with PEG-200, glycerol or polypropylene glycol (PPG), molecular weight 425 g/mol (German Patent Application No. DE 42 19 368).
The ideal performance characteristics of an adhesive composition intended for use on human skin and/or mucosal tissue present difficult and conflicting technical requirements. Initially, the adhesive should be suitable for long-term skin contact, and permeable to and physically and chemically compatible with any active agent and any permeation enhancers or other vehicles or additives that are present. The ideal adhesive should also be nonirritating, noncomedogenic and nonsensitizing, yet bond quickly to skin or mucosal tissue at the intended site of use with only very slight pressure. The adhesive should maintain its bond for as long a period of time as necessary and be resistant to inadvertent removal, yet be easily removed without removing any skin or leaving a residue (a suitable strength of an adhesive joint with the skin ranges from about 200 to 400 N/m under the 180 degree peel test). Furthermore, the adhesive composition should not be sensitive to or degradable by exposure to moisture or high humidity.
With bioadhesives, hydrophilic compositions are preferred for the adhesive compositions to adhere well to moist substrates. Hydrophilic adhesives are advantageous in other respects as well, insofar as:
(1) hydrophilic adhesives can provide greater adhesion compared with hydrophobic adhesives, because the surface energy of hydrophilic adhesives is typically higher and closer to that of biological substrates such as skin and mucosal membranes;
(2) hydrophilic adhesives are compatible with a wide variety of drugs, excipients and additives;
(3) the plasticizing effect of water sorbed by hydrophilic adhesives from hydrated skin or mucosal tissues enhances adhesion, in contrast to hydrophobic adhesives;
(4) the enhanced solubility of drugs in hydrophilic adhesives facilitates control over drug release kinetics;
(5) with hydrophilic adhesives, based on hydrophilic polymers, there is an expanded capability to control and manipulate the adhesive-cohesive balance; and
(6) the adhesive properties of hydrophilic polymers are considerably less sensitive to their molecular weight than those of hydrophobic polymers, as a result of specific intramolecular and intermolecular interaction within hydrophilic adhesives.
In order to increase the hydrophilicity of an adhesive composition, hydrophobic PSAs have been “hydrophilized” by incorporation of non-tacky hydrophilic polymers and fillers into a hydrophobic adhesive. Thus, polyisobutylene (PIB) PSA has been hydrophilized by incorporation of cellulose and cellulose derivatives (U.S. Pat. No. 4,231,369), polyvinyl alcohol (PVA), pectin and gelatin (U.S. Pat. Nos. 4, 367,732 and 4,867,748), and SiO2 (U.S. Pat. No. 5,643,187). Rubber adhesives have also been modified by filling with amphiphilic surfactants, or by treating the PSA polymer with a plasma-oxygen discharge. Acrylic PSAs can be hydrophilized by incorporation of PVP (U.S. Pat. No. 5,645,855). Hydrophilization of hydrophobic adhesives, while somewhat effective, tends to result in a partial loss of adhesion.
There is, accordingly, a need in the art for improved bioadhesive compositions that combine the properties of hydrophobic PSAs with the many advantages of hydrophilic adhesive compositions. It would also be ideal if such an adhesive composition could be adapted for a variety of uses, e.g., in wound healing and bandages, in the fabrication of transdermal and other drug delivery systems, in preparing medicated adhesive formulations for topical and transdermal pharmaceutical formulations, in pressure-relieving cushions (which may or may not be medicated), as sealants for ostomy devices and prostheses, as conductive adhesives for attachment of electroconductive articles such as electrodes to the skin, and the like.